What You Must Know About Motors
If you’re an electrical engineer, you can stop reading this article right now. This story isn’t for the gear heads — it’s for the rest of you woodworkers who use power tools every day but are occasionally stupefied by amps, volts, watts and horsepower. I’ll warn you, there’s just the tiniest bit of math to learn here. But if you can multiply and divide two numbers, you will open up a whole new world of understanding when it comes to the subject of motors.
The first thing to understand is that there are two kinds of motors that power almost all of the machinery in a home workshop: induction motors and universal motors. Each type has its strengths and weaknesses. The reason that you need to know the difference between the two is that some tools (table saws, planers and jointers, for example) can be powered by either type of motor. So you need to educate yourself so you’ll choose the right motor for the kind of work you do.
In general, induction motors power stationary machinery that must run for hours on end, such as big table saws, planers, band saws and jointers. Universal motors power mostly hand-held stuff: routers, jigsaws and sanders. However, this is changing. These days you’ll find more and more universal motors in benchtop table saws, small jointers, spindle sanders and portable planers.
I like to think of the two motors as the tortoise and the hare. Induction motors are the tortoise of the pair. They’re rugged, quiet, large, heavy, turn more slowly and can be stalled under heavy use. They are great for the long haul. Universal motors, on the other hand, have a shorter life span, they’re smaller, they make more noise, they operate at very high speeds, they offer the most horsepower per pound of any alternating current motor, and they are very difficult to stall. Universal motors provide large amounts of power in quick bursts with constant torque and at variable speeds.
It might help to think about how you use tools with universal motors. If you’ve got a chop saw, you need a burst of power for three or four seconds to make your cut. You need torque and you need it fast. Same goes for biscuit joiners and routers. Unless you are running parts for 100 doors on your router table, chances are that these tools are on for five minutes and then off for a while. Now think about how you use a jointer or a planer with a hefty induction motor. You might have 100 board feet of lumber to surface. Each board might have to go through that machine five times. Your machine might be running for hours on end.
So each type of motor has a type of job that it’s really good at. And it all has to do with the way that the motor is built. Here’s the inside story:
The reason they are called “induction” motors is the way they convert electricity into a spinning rotor. To understand how induction motors work, let’s say you’ve got one of these puppies in your table saw and you’re about to turn it on. As you flip the switch, power flows into what’s called the “stator” and magnetizes it. The stator is a mass of copper windings that surround the rotor in the center, which is what spins the saw’s blade through a series of belts and pulleys. Inside the stator are two or four “poles” that become magnetically charged because of the electricity running through the wires. When the electricity changes direction or cycles, as it does 60 times a second in the United States (hence the term 60 cycles), each pole changes its magnetic strength, from a positive to a negative value or from a negative to a positive value.
The induced poles in the rotor are then attracted and repulsed by these ever-changing electromagnets in the surrounding stator. The motor isn’t running, but the rotor is excited. What this hulk of iron and copper now needs is a shot of power from another copper winding (called a “starting winding”) that is out of phase physically and electrically with the main winding. And that’s where the capacitor comes in. In most modern tools a capacitor (which is in series with the “starting winding”) helps with the starting torque. Then, when the motor reaches 85 percent of its speed, the capacitor and the starting winding drop out of the circuit and the motor runs on its main winding.
Whew. So, this is the long way to explain why these are called induction motors. As you can see, the rotor spins because it is “induced” by the electromagnets in the stator. Induction motors are large and heavy because the induction process takes a lot of iron and copper (a ? hp induction motor weighs about 25 pounds; a ? hp universal motor weighs 2? pounds). Induction motors are reliable because they’re simple, their parts are built for long life and they run at slow speeds (so they don’t generate as much motor-damaging heat). In fact, a well-built induction motor won’t heat up more than 40 degrees centigrade over room temperature. Induction motors are slow because the rotations per minute (rpms) are governed by how many poles are inside the stator and the number of times per second that your electricity cycles — which is standard at 60 cycles. So now you can understand why you wouldn’t want your router powered by an induction motor — you could barely lift it, and it probably would be too slow and not have enough torque.
Universal motors get their name from the fact that many of them can operate on both alternating current (from an outlet) or direct current. The way that universal motors work is a little more complicated than their induction cousins, but there are similarities.
Instead of a rotor, universal motors have what’s called an armature that spins in the center. Instead of a stator, universal motors have what’s called a field, usually consisting of two coils surrounding the armature. Universal motors also have some parts that induction motors don’t. On one end of the armature is a part called the commutator. This part is round like the armature, but it is usually smaller in diameter and is made of small bars of copper. It’s through these bars that the armature winding is energized. Universal motors also have what are called “brushes.” Brushes are made from a carbon-graphite material and are usually held in place against the commutator by small springs. When you turn on a universal motor, current travels in what’s called a “series circuit.” One side of the electrical line goes through the field, then through the brushes, into the commutator, then the armature, and back to the other side of the line. Each of the bars in the commutator changes polarity as it contacts a brush, and this changes the polarity in the magnets in the armature. The magnetic forces in the armature react with the electromagnets in the field coils and the motor develops torque.
Universal motors make a lot of noise because they spin at a dizzying speed — sometimes seven times faster than an induction motor — and their fans suck a lot of air through the motor, which makes noise. Universal motors are less reliable for three reasons. The motor generates more heat, which can cause the components to break down. Second, the carbon brushes wear out. If they can be replaced then it’s a quick fix. If they can’t, you’ve got trouble. And third, the big fan that cools the motor brings in a lot of junk such as sawdust and foreign objects. This junk can damage the windings and insulation.